A gyrometer is a motion sensor which makes it possible to measure the rotation rate of the reference frame of the sensor with respect to a Galilean reference frame, about one or more axes.
A laser gyrometer, or laser gyro, is a bidirectional ring laser which makes it possible to measure an angular rate (or a relative angular position by integration over time). It consists of an optical cavity composed of several mirrors assembled on a block in which paths are drilled to provide an optical cavity. An amplifying medium is inserted into the optical path of the cavity and an excitation system must provide it energy making it possible to generate the laser gain. The elements from which the laser cavity is composed are chosen so as to allow bidirectional operation: the laser cavity must be able to simultaneously sustain two waves which propagate in opposite directions (so-called counter-rotating waves).
Laser gyros are generally used for the measurement of rotation rates or angular positions. This type of equipment is notably employed for aeronautical applications.
The operating principle of a laser gyro is based on the Sagnac effect in a ring laser cavity to which a rotational motion is imparted. When the cavity is stationary, the two counter-rotating waves exhibit the same optical frequency. In the presence of a rotational motion in the plane of the optical cavity, the Sagnac effect induces a difference of frequency Ω between the two counter-rotating optical waves. A fraction of the energy of each wave is extracted from the cavity. A recombination device causes the two extracted beams to interfere so as to form interference fringes which are observed with the aid of one or more photo-detectors. In an ideal laser gyrometer, the frequency of the fringes in front of the photo-detector is proportional to the rotation rate impressed on the cavity and their direction of travel depends on the direction of rotation.
The majority of laser gyrometers, also called laser gyros, use a gaseous amplifying medium which is customarily a mixture of helium and neon. The excitation of the gaseous amplifying medium is then generally performed by creating a plasma in the gas, for example by generating a discharge between two electrodes which are mechanically bound securely to the cavity. However, the gaseous nature of the amplifying medium remains a source of technical complications during the production of the laser gyro, notably because of the high gas purity required. Furthermore, it induces sources of aging of the laser, with sensitivity to leak-tightness of the cavity, degradation of certain electrodes with operation.
It is possible to produce a laser gyro with solid-state amplifying medium, in which the gaseous amplifying medium is replaced with a solid element, for example, by employing Neodymium ions in a YAG (Yttrium-Aluminum-Garnet) matrix, commonly denoted by Nd:YAG, it is possible to produce a solid-state laser gyro operating in the near infra-red. A crystalline matrix or a glass doped with ions belonging to the class of the rare earths (Erbium, Ytterbium, etc.) or else a semi-conducting material can be used as amplifying medium. All the problems inherent with the gaseous state of the amplifying medium are thus obviated. Since the crystalline or glass matrices commonly employed are very bad electrical conductors, only optical pumping can excite the amplifying medium. An optical beam of appropriate wavelength must be injected into the useful volume of the solid amplifying medium so as to induce the population inversion of the desired atomic transition which makes it possible to induce the optical gain. This pumping can currently be carried out effectively with the aid of laser diode or fibered laser diodes.
Solid-state laser gyros are generally used to measure rotation rates or angular positions. This type of equipment is notably employed for aeronautical applications.
To optimize the optical pumping, it is possible to perform a longitudinal pumping, or, stated otherwise, to inject energy into the axis of the resonant beam, through a mirror of the laser cavity. Additional internal devices can optionally be employed to stabilize a counter-rotating wave manner of operation. A measurement of the rotation rate of the cavity can then be performed by observing the interferences between the two counter-rotating resonant waves.
The longitudinal configuration of the optical pumping makes it necessary to superimpose the injected beam on the optically stable beam in the cavity with a precision of less than twenty micrometers. Such precision is customarily obtained by securely binding the optical components of the injection to an independent support furnished with elements for adjusting position and tilt. The motion of this support is not tied to that of the optical cavity.
In order to compensate for the limits inherent in laser gyros with a gaseous-state amplifying medium, for low rotation rates, typically rotation rates of less than a few tenths or indeed a few hundredths of a degree per second, it is generally chosen to permanently subject the cavity to an oscillating rotational motion, by mechanical activation, at a frequency of the order of 100 Hz to 1 kHz. This mode of implementation allows laser gyros with gaseous-state amplifying medium to operate correctly in this low rotation rate range termed the blind zone which is due to backscattering from the mirrors. The excitation device, generally consisting of two electrodes between which a large difference of electrical voltage causes an ionization a gas, is bound securely to the block and it remains entrained in the activation of the cavity.
A laser gyrometer with solid-state amplifying medium, for example a Neodymium-doped YAG matrix, exhibits the same blind zone limit, which may be increased on account of an additional coupling of the counter-rotating waves in the solid-state amplifying medium, which generally has a so-called homogeneously broadened gain. Transposing such a mode of implementation, i.e. mechanical activation of the cavity, to a solid-state laser gyro poses a problem, since in this instance, the injection optics is external to the cavity which experiences an oscillating rotational motion that temporally modifies the position of the amplifying medium with respect to the pumping beam, thereby causing modulations of the intensity emitted by the cavity that may give rise to severe malfunctioning of the laser gyro.